vmcoreinfo: warn if we exceed vmcoreinfo data size

Though vmcoreinfo is intended to be small, at just one page, useful
information is still added to it, so we risk running out of space. 
Currently there is no runtime check to see whether the vmcoreinfo buffer
has been exhausted.  Add a warning for this case.

Currently, my static checking tool[1] indicates that a good upper bound
for vmcoreinfo size is currently 3415 bytes, but the best time to add
warnings is before the risk becomes too high.

[1] https://github.com/brenns10/kernel_stuff/blob/master/vmcoreinfosize/vmcoreinfosize.py

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Stephen Brennan <[email protected]>
Acked-by: Baoquan He <[email protected]>
Cc: Dave Young <[email protected]>
Cc: Vivek Goyal <[email protected]>
Cc: Kees Cook <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: brenns10

kernel/crash_core.c

@@ -383,6 +383,9 @@ void vmcoreinfo_append_str(const char *fmt, ...)
 	memcpy(&vmcoreinfo_data[vmcoreinfo_size], buf, r);
 
 	vmcoreinfo_size += r;
+
+	WARN_ONCE(vmcoreinfo_size == VMCOREINFO_BYTES,
+		  "vmcoreinfo data exceeds allocated size, truncating");
 }
 
 /*